Nanocrystals produce hydrogen using sunlight

Researchers in the US have made hydrogen fuel using just sunlight, nanocrystals and a cheap nickel catalyst. The new artificial photosynthesis process is the first of its kind to continually produce fuel for several weeks without slowing down. As a result it could be important for green-energy applications and also for certain industrial processes such as those for producing ammonia.

During photosynthesis, plants harness solar radiation and convert it into energy. Most artificial photosynthesis systems try to mimic this natural process by exploiting light-absorbing dye molecules called chromophores to split water into hydrogen and oxygen. The hydrogen is produced in the reductive side of the reaction and the oxygen in the oxidative side. These so-called half-reactions are part of the process that converts light into energy, but the problem is that such technologies are inefficient and short-lived because the Sun's rays damage and destroy the light-absorbing dyes in just a few hours.

Now, a team of researchers led by Todd Krauss, Patrick Holland and Richard Eisenberg at the University of Rochester has developed a new photochemical hydrogen-generating system made of cadmium–selenide (CdSe) quantum dots, nickel salt catalysts and ascorbic acid (vitamin C). The system lasts for several weeks rather than just hours and, in water, has an quantum efficiency of 36% – for every 100 photons absorbed, 36 hydrogen molecules are produced. If the surrounding solution is a mix of water and ethanol, this efficiency increases to 66%. Such high values have never yet been observed for such all-solution-based systems. The only snag is that the vitamin C (which acts as an electron donor) gets used up and regularly needs to be replenished during each hydrogen production cycle.

How it works

The CdSe quantum dots absorb two photons of light and transfer two electrons to the nickel catalyst. The two remaining protons combine to produce a hydrogen molecule, explains Krauss. "Our work is different from most other previous research in that the catalyst is formed in situ from the quantum-dot ligands," he says. "Most other solution-based systems produce hydrogen for just hours, or at most a day, because the chromophores degrade, so our long-lived system is rather unusual."

The researchers say that their catalyst–nanocrystal pairs are better than previous artificial photosynthesis nanoparticle systems because they are more stable to sunlight, but admit that they do not yet know why this is the case.

"This new system will also certainly help us better understand the reductive side of artificial photosynthesis – something that may one day help lead to more effective and efficient water splitting," adds Krauss, "Our work is an important step in that direction."

Making ammonia

According to the team, such a clean source of hydrogen could not only find applications in green energy, but also in industry, for example in the Haber process for producing ammonia.

The Rochester team is now looking at other nanoparticle systems to try out. "We are also investigating other less-expensive catalysts and hope to find a way to replace the sacrificial vitamin C molecule with electrons, say from a circuit. Such experiments could be the next step towards a true artificial photosynthesis system, but we are still a far cry from that since we have only performed half of the full reaction," says Krauss.

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8 comments

way to go!

Nice, is good to see that they not only found this but are also trying to avoid the use of vitamin C and the catalyst for a more straight forward way from sun to hydrogen. I hope they find out what has given those nanocrystals it's resistance otherwise it will be a lost cause to continue further.

nanocrystals produce hydrogen using sunlight

Think of the implications for fuel cells and solar panels (thin-film variety). Combining the two, could we produce a 'machine' that helps alleviate the ozone hole problem over the Anartica by pumping up O3 generated by massive (amounts of) fuel cells in the Indian Ocean south of Australia. i.e. funneling effect-'the tiniest titan' june 2012 National Geographic.

Not the catalyst

Nice, is good to see that they not only found this but are also trying to avoid the use of vitamin C and the catalyst for a more straight forward way from sun to hydrogen. I hope they find out what has given those nanocrystals it's resistance otherwise it will be a lost cause to continue further.

Clarification regarding efficiency

A reader has pointed out that our use of the term "efficiency" could be confusing. When discussing solar cells that convert light to electrical energy (which is not the subject of this article), efficiency is defined as electrical power out divided by solar power in. In this article, efficiency is the number of hydrogen molecules produced divided by the number of photons absorbed. Chemists call this "quantum yield" or "quantum efficiency", and so I have edited the article to include the latter term.

hydrogen

Nanocrystals produce hydrogen using sunlight.

Using a different approach and avoiding the use of short lived organic dyes is brilliant. Even though the nickle catalyst is sort of costly it does not get used up, it seems, and so a little should go a long way. Now the next step is to scale it up and work on the longevity some more. Solar powered production of hydrogen gas from water is a way to allow solar power to produce energy all day and all night. That is the long awaited answer. So now we need to have work done advancing the concept to produce practical versions for commercial use.